A microscope is one of the best process control tools an operator can have, and one of the least used. Many plants own one that sits in a cabinet because nobody is sure what to look for. You do not need to be a microbiologist to get value from it. A simple, consistent routine of 10 to 15 minutes a day, recorded the same way every time, will show you changes in the sludge well before they show up in the lab data.
This article lays out that routine. The microscopy library has a page for each organism group with line sketches and notes.
What the microscope can and cannot tell you
It can tell you:
- Whether floc is strong, weak, compact, open, large or small.
- Whether filaments are present and how many.
- What kinds of protozoa and small animals are present, which reflects sludge age, DO and stability.
- Whether something has changed since yesterday.
- Early signs of toxicity.
It cannot tell you:
- Exactly what SRT or F/M you have. Use your data for that.
- The exact species of a filament without staining and experience.
- Anything definitive from a single look. Trends matter.
Equipment
- A compound microscope with 10x, 40x and ideally 100x (oil) objectives. Phase contrast is a big help for seeing protozoa and filaments.
- Clean glass slides and cover slips.
- A dropper or pipette.
- Gram and Neisser stain kits if you want to identify filaments.
- A phone adapter or camera for photos (very useful for a record).
Preparing a wet mount
- Take a fresh sample from the aeration basin effluent, the same place as the settleometer.
- Mix gently and place one drop on a slide.
- Lower a cover slip at an angle so it does not trap air bubbles.
- The liquid should spread to the edges without overflowing. Too much liquid makes everything drift; too little crushes the floc.
- Look within 10 to 15 minutes. Organisms slow down and change as the slide warms and runs out of oxygen.
Step 1: Scan at 100x
Start with the 10x objective (100x total magnification). Move the slide in a pattern so you cover the whole cover slip, not just the center.
Floc
Look at several dozen flocs and note:
- Size: small (less than 150 microns), medium, or large (more than 500 microns).
- Shape: round and compact, or irregular and open.
- Strength: firm with clear edges, or loose and breaking apart.
- Dispersed bacteria: clear background, or cloudy with many single cells between flocs.
Healthy floc is medium to large, irregular, firm, with a few short filaments inside acting as a backbone and a clear background.
Filaments
Rate filament abundance on a 0 to 6 scale:
- 0 None
- 1 Few (present in some flocs)
- 2 Some (common in some flocs)
- 3 Common (in most flocs, low density)
- 4 Very common (most flocs, medium density)
- 5 Abundant (most flocs, high density)
- 6 Excessive (more filaments than floc)
Abundance of 4 or more usually affects settling. Note whether filaments stay inside the floc or extend outward and bridge between flocs. Bridging filaments hurt settling most.
Protozoa and small animals
Note which groups are present and roughly how many. You will learn to recognize them quickly at 100x and confirm at 400x.
Step 2: Look closer at 400x
Switch to the 40x objective. Look at:
- Protozoa: stalked ciliates, free-swimming ciliates, crawling ciliates, amoebae, flagellates.
- Small animals: rotifers, nematodes, water bears, worms.
- Filament features: thickness, length, straight or curved, branched or not, sheath, attached growth, cell shape.
Indicator organisms
Protozoa and small animals are useful indicators because different groups thrive in different conditions. They are not precise instruments, but changes in the population tell you something has changed.
Amoebae and flagellates dominate young sludge, startup, and recovery after upsets. Large numbers can mean high F/M, low SRT, low DO, or a recent toxic event. See amoebae and flagellates.
Free-swimming ciliates increase as the sludge matures and there are plenty of dispersed bacteria to eat. Many free swimmers with a cloudy background suggests dispersed growth. See free-swimming ciliates.
Crawling and stalked ciliates graze on floc and indicate a stable, well settled sludge with good effluent quality. Stalked ciliates such as Vorticella attach to floc; colonial forms like Epistylis and Opercularia are common at longer SRT. Stalked ciliates that are closed up or detached can be an early sign of toxicity or low DO. See stalked ciliates.
Rotifers indicate an older, stable sludge with good floc. Very high numbers can come with very long SRT. See rotifers.
Nematodes and water bears suggest a long SRT and stable conditions. See nematodes.
A rough sequence from young to old sludge is: amoebae and flagellates, then free-swimming ciliates, then crawling and stalked ciliates, then rotifers and nematodes. A healthy conventional plant usually shows a mix with stalked and crawling ciliates prominent.
Filament identification
If filaments are a problem, identifying the type points to a probable cause:
- Low DO: S. natans, Type 1701, H. hydrossis.
- Low F/M: M. parvicella, Types 0041, 0675, 1851, 0092.
- Septicity and sulfide: Thiothrix, Type 021N, Beggiatoa.
- Nutrient deficiency: Thiothrix, Type 021N.
- Grease: Nocardioforms, M. parvicella.
Identification uses the Gram stain, the Neisser stain, the sulfur test, and features at 1,000x. It takes practice. Our filamentous organisms page is a starting point; reference manuals and training help a great deal.
Step 3: Record and connect
Use the same form every day:
- Date, time, sample location.
- Floc size, shape, strength, background clarity.
- Filament abundance (0 to 6) and dominant type if known.
- Protozoa and animals present, with a rough count or a "few, common, many" scale.
- Photos.
- Notes on anything unusual.
Then connect it to your process data. If filament abundance rises from 2 to 4 over a week, check DO trends, F/M, nutrients and septicity. If stalked ciliates disappear overnight and the background goes cloudy, check for a toxic discharge. If rotifers and nematodes increase while floc gets small and dense, check whether SRT has crept up.
Example: White River WRRF
At the fictional White River WRRF, the September blower outage left DO near 1.0 mg/L from September 7 to 12. On the operator's daily microscope log, filament abundance went from 2 (some) to 4 (very common) in five days, and the dominant filaments were sheathed, straight and Gram negative with false branching, consistent with S. natans, along with Type 1701. Stalked ciliates remained present, which suggested no toxicity. That combination (low DO filaments, stable protozoa) matched the DO data and pointed straight at aeration capacity as the cause.
Getting started
If you have never used the microscope, start simple:
- Look at one slide a day at 100x for two weeks. Just draw or photograph what you see.
- Learn floc and filament abundance first.
- Learn the main protozoa groups next.
- Add staining once you are comfortable.
Use the Identify what I'm seeing questionnaire to work through an observation step by step. Within a month you will have a baseline, and once you know what "normal" looks like for your plant, changes stand out.